The present invention relates to a semiconductor device and a method for fabricating the semiconductor device, more specifically to a semiconductor device including an MIS (Metal-Insulator-Semiconductor) transistor having a gate insulating film of a high dielectric constant film, and a method for fabricating the semiconductor device.
As MIS transistors are increasingly downsized due to higher integration of semiconductor devices, the gate insulating films are made increasingly thinner. Transistors for gate lengths of below 50 nm are expected several years later. Gate insulating films of film thicknesses of below 1 nm in terms of the thickness of the silicon oxide film are required.
As the gate insulating films, conventionally silicon oxide-based insulating films have been dominantly used. However, there has been noted a problem that when the silicon oxide-based insulating films have thicknesses of below about 3 nm, the tunneling leakage current is conspicuous, and the silicon oxide-based insulating films fail to function as the insulating film. Studies are being made of forming the gate insulating films of new materials taking the place of the silicon oxide-based insulating films, whose thicknesses are below 1 nm in terms of the thickness of the silicon oxide-based insulating films.
It is being studied to use materials whose dielectric constants are higher than the dielectric constant of silicon oxide (high-k materials) as a gate insulating film in place of the silicon oxide-based insulating films. The use of the high-k materials permits the physical film thickness of the gate insulating film to be thick, whereby the leakage current can be suppressed.
As the high-k materials, various metal oxide materials are proposed; ZrO2, Al2O3, HfO2, TaO2, etc. are noted. Among them, HfO2 is prospective because of advantages that the relative dielectric constant is about 20˜30, which is high; HfO2 is not easily silicidized more than ZrO2; that the interfacial layer thereof with respect to a silicon substrate does not much increase while being grown; etc.
However, as-grown HfO2 is partially crystallized and disadvantageously has large leakage current. It is known that when polycrystalline silicon film is grown on HfO2, the polycrystalline silicon locally abnormally grows.
As for the problem of the crystallization of HfO2, it is proposed to mix a non-crystalline material in the gate insulating film as described in, e.g., Reference 1 (Japanese published unexamined patent application No. 2001-267566). As described in Reference 2 (Japanese published unexamined patent application No. 2002-033320), it is proposed to mix SiO2, Al2O3 or others, which are not easily crystallized into HfO2 to thereby suppress the crystallization so as to suppress the leakage current. As for the abnormal growth of the polycrystalline silicon, Reference 3 (D. C. Gilmer et al., “Compatibility of polycrystalline silicon gate deposition with HfO2and Al2O3/HfO2 gate dielectrics”, Appl. Phys. Lett. Vol. 81, pp. 1288-1290 (2002)) discloses that an AlO3 film is formed between an HfO2 film and a polycrystalline silicon film to thereby suppress the abnormal growth of the polycrystalline silicon.